High-precision-resolution image acquisition apparatus and method

ABSTRACT

A method and apparatus for setting and repeatably and reliably maintaining a desired image resolution. The invention features an optical, image acquiring device and a pair of light sources, the beams from which are caused to converge at a distance from the optical device which provides the desired resolution. Subsequently, positioning the optical device with the point of convergence of the beams on the surface of an object being imaged ensures the desired resolution. In preferred embodiments, the plane yielding the desired resolution is caused to be coincident with the plane of optimal focus.

This application claims benefit of Provisional application Ser. No.60/042,525 filed Apr. 1, 1997.

FIELD OF THE INVENTION

The invention relates in general to image acquisition using a cameraand, in particular, to an apparatus and method used to set and maintainthe resolution of an image with a high degree of precision.

BACKGROUND OF THE INVENTION

In certain situations, it is necessary or desirable to know and to beable to reproduce repeatedly and reliably the resolution of an imagecaptured on a medium. In the context of an image captured on film, i.e.,as a conventional photograph, the term “resolution” is analogous to thescale of the photograph and indicates how a distance or dimensionmeasured “off of” the photograph corresponds to the distance ordimension in the scene or object that has been photographed. In thecontext of digital image acquisition using, e.g., a digital“photographic” or “still” camera or a digital video camera, the term“resolution” refers to the number of pixels in the image correspondingto a distance or dimension in the scene or oject being “photographed” ormonitored, typically referred to as image pixels per object inch or,more conveniently, dots per inch (“dpi”).

One such application where it is necessary to know the resolution of animage (object inch per photograph inch in a film-based photograph ordots per object inch (dpi) in a digital image) relates to photographingfingerprints, e.g., for purposes of crime investigation. Whenfingerprints are photographed using a conventional, film-based camera,the person taking the picture typically puts an object of known size(“reference object”), e.g., a ruler or a coin, in the field of view withthe fingerprint target. Dimensions within the fingerprint, e.g., fromone fingerprint landmark to another, are then calculated by measuringthe distance as shown in the photograph and scaling that distance (asshown in the photograph) either up or down, the scale factor being equalto the known, actual size of the reference object divided by the size ofthe reference object as shown in the photograph. Similarly, if a digitalcamera is used (either still (“photographic”) or video), the relevantdistance is determined by measuring it in the image in terms of pixels,and then multiplying the number of pixels by an appropriate scale factorexpressed in terms of length per pixels, the scale factor being equal toa known dimension of the reference object divided by the number ofpixels in the image corresponding to that known dimension.

Calibrating the resolution of the image in this manner can betime-consuming, difficult, and therefore inaccurate. Accordingly, thereis a need for an apparatus and method to facilitate recording an imagewith a known resolution, and doing so repeatedly and reliably.

SUMMARY OF THE INVENTION

The present invention fulfills this need. In general, the inventionprovides an apparatus and method by which a camera—preferably a digitalvideo camera—can be positioned with respect to an object being imaged,with the resolution consistently and easily being maintained at adesired value. Furthermore, in preferred embodiments of the invention,the camera can be positioned such that the plane of optimal focus,typically at the center of the depth of field, is coincident with theplane of view having the required resolution.

In general, this is achieved with an apparatus in which the distancefrom the camera to a target object can be adjusted and then held fixed.The apparatus uses two or more light sources, e.g., lasers, collimatedlights, spotbeams, slit lamps, etc., whose light axes can be pivotedsuch that their beams of light intersect at the surface of the targetobject. The light sources, which are held at fixed distances from thecamera, can be locked into the particular angular orientation withrespect to the camera. With this apparatus, once the resolution is set,that resolution can be regained for any subsequent object placed infront of the camera, regardless of its dimension or size, simply bymoving the camera with respect to the new object until the light beamsonce again intersect at the surface of the new object. Alternatively, asingle light source can be used and the camera is moved with respect tothe new object until the point at which the light beam strikes the newobject is at the same location within the field of view of the camera.

The apparatus includes a support surface which supports an object thatis to be imaged, and the camera is supported at an optical distance fromthe support surface (i.e., from the object). The optical distance fromthe object to the camera is adjustable such that the resolution of animage of the object can be adjusted. The apparatus further includes alight source which projects a beam of light onto the object. The lightsource is configured such that the location where the beam of lightstrikes the object varies as the optical distance is varied.

Preferred embodiments of the invention may include one or more of thefollowing features. Preferably, the apparatus is configured such thatthe beam of light is angled relative to the optical path (axis) alongwhich the image of the object being observed is acquired. Preferably,the apparatus includes two light sources, both of which may be angledrelative to the optical path such that the beams of light projectedthereby can be rotated so as to intersect at the surface of the objectbeing imaged.

Preferably, the light source or sources is or are lasers, and where twolasers are used, it is preferable for one of them to be a line laserwith the other being a more conventional laser which projects a dot orspot of light. This permits the two beams to be distinguished moreeasily, thereby facilitating adjustment of the optical distance based onthe relative positions of where the beams strike the surface of theobject.

Preferably, the apparatus includes computer hardware and software foranalyzing a signal generated and output by the camera. The signal can beanalyzed to determine the resolution of the image acquired by thecamera, which resolution may be expressed in terms of pixels per unit oflength of the object being imaged (dpi).

In further contemplated embodiments, the apparatus includes a motorwhich adjusts the optical distance between the camera and the supportsurface and/or motors which adjust the angle of the light source orsources relative to the optical path. With such motors, the image can beanalyzed by computer and the motors can be controlled by the computer toadjust a) the optical distance (whether to achieve a desired resolutionduring calibration or to return to that desired resolutionsubsequently); and/or b) the angle of the light beam or beams relativeto the optical path such that a single beam strikes the surface of anobject being imaged at a desired location (e.g., in the center of thefield of view) or such that two beams converge at the surface of theobject being imaged.

In still further contemplated embodiments, the apparatus may include andx-y-θ support platform to facilitate precise positioning of the objectbeing imaged. Furthermore, the light sources preferably are interlockedwith the camera such that they are turned off when the image of anobject is actually being acquired.

In another aspect, the invention features a method for acquiring theimage of an object with a repeatable resolution. The method includes thesteps of providing a camera at an optical distance from an object, theobject being positioned within the field of view of the camera. Firstand second light sources are provided and project beams of light on thesurface of the object. One or both of the light sources are adjustedsuch that the beams of light converge on the surface of the object. Theobject is then removed from the field of view and a second object ispositioned within the field of view of the camera. The optical distancefrom the camera to the second object is adjusted, as necessary, suchthat the first and second beams of light converge at the surface of thesecond object. This guarantees the same resolution of an image of thesecond object as the resolution of an image of the first object wouldhave been. An image of the second object is then acquired.

In yet another aspect, the invention features a method of acquiring theimage of an object with a repeatable resolution. In this aspect, thepositioning of the camera (and hence the resolution of the image) isadjusted based on where in the field of view of the camera the pointwhere a beam of light strikes the object lies. This may be done using asingle light source and adjusting the optical distance such that thebeam of light projected by the single light source strikes the surfaceof an object being imaged precisely in the center of the field of viewof the camera, or using two light sources and adjusting the opticaldistance such that the two light sources converge at the surface of theobject being imaged, the location of the point of convergence lying atthe same position within the field of view.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described inconnection with the following drawings, in which:

FIG. 1 is a schematic, perspective view showing ahigh-precision-resolution image acquisition apparatus according to theinvention;

FIG. 2 is a schematic front view, along the lines 2—2 in FIG. 1, of anapparatus according to the invention;

FIG. 3 is a plan view showing the calibration pattern of the calibrationtarget shown in FIG. 1;

FIG. 4 is a detail view showing the cross of the calibration patternshown in FIG. 3, with the beams of a spot laser and a line laserconverging centrally thereon;

FIG. 5 is a schematic, perspective view showing a calibration blockwhich may be used instead of the calibration target of FIG. 1; FIG. 5Ais a plan view showing the calibration pattern on the inclined surfaceof the calibration block shown in FIG. 5; and FIG. 5B shows the image ofthe calibration pattern shown in FIG. 5A, with the central portionthereof in focus and end portions thereof out of focus;

FIGS. 6A and 6B are a schematic, side elevation view and a schematic,plan view, respectively, of a horizontally oriented alternativeembodiment of the apparatus shown in FIG. 1; and

FIG. 7 is a schematic, perspective view, broken away, showing an x-y-θtable incorporated into the embodiment of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

An image acquisition apparatus 10 according to the invention is shown inFIG. 1. The apparatus includes a support stand 12, which consists of aflat platform 14 and a camera support post 16. Camera 18 is securelyattached to laser mounting bracket 20 via a separate camera “standoff”or support bracket (not visible), with camera lens 22 aimed downwardtoward the platform 14. In a preferred embodiment, the camera 18 is aKodak® MASD MEGAPLUS™ 1.4 i model digital video camera, available fromthe Motion Analysis Systems Division of the Eastman Kodak Company, andlens 22 is a Componon-S 50 millimeter/F2.8 lens, available fromSchneider Optics, Inc. in Hauppauge, N.Y., preferably used with anextension tube 23. The camera outputs an image signal that is sent toframe grabber/central processing unit 19, as described in greater detailbelow, and video display/user interface 21 displays an image of theobject being viewed and allows a user to use the system. The lasermounting bracket 20, along with camera 18, is supported away from thesupport post 16 by support arm 24. Support arm 24 is securely connectedto (or may be formed integrally with) and extends from collar 26. Collar26 fits over support post 16 and moves up and down along it, therebyvarying the distance from the camera 18 to the platform 14. Preferably,a locking device (not shown) such as a large set screw (e.g.

A pinion gear 30 extends through the collar 26 to mate with rack gear 32formed along the side of support post 16. Pinion gear 30 is turned usinghand knob 34, thereby raising or lowering the camera assembly. In analternative embodiment, depending on the materials used to construct thesupport post 16, rack gear 32 may be eliminated and friction rollersused instead of pinion gear 30.

In another alternative embodiment, an appropriate drive motor 36,indicated schematically in phantom, may be provided such that raisingand lowering of the camera assembly is motorized. In such embodiment,position encoders (not shown) can also be provided. The encoders wouldprovide information to the central processing unit (CPU) or to someother controller, and the CPU or controller would control the verticalpositioning of the camera, particularly by monitoring the position andmovement of the laser beams (described below) in the image, which isdigitized and passed to the CPU as described below.

As further shown in FIG. 1, first and second light sources 38, 40 aremounted at the opposite ends of the laser mounting bracket 20. In apreferred embodiment, the light sources 38, 40 are lasers, one of whichpreferably is a line laser and the other of which is a more conventionallaser which projects a dot of laser light on the surface beingilluminated. If this is so, it is preferable for the line laser to bemounted such that the line cast by the laser beam extends parallel totwo of the edges of the field of view of the camera 18 and perpendicularto the other two edges of the field of view (assuming a square orrectangular field of view), with the line cast by the line laser beingoriented perpendicular to the projection onto the base 14 of a lineconnecting the two laser light sources such that the laser line may beseen to move laterally as the camera assembly is moved up and down. Inalternative embodiments, the light sources 38, 40 may be collimatedlights, spot beams, slit lamps, etc.

As shown in FIG. 2, light sources 38, 40 are pivotally mounted at theends of laser mounting bracket 20 on mounting brackets 42, 44,respectively. By pivoting the light sources 38, 40, the angles θ and Φwhich laser beams 46, 48 make with respect to the optical axis (path) 50of the camera are varied, and the location of the point of intersection52 of the two beams 46, 48 is changed. The lasers should also bepivotable in their respective planes which are oriented perpendicularlyto the plane of FIG. 2 and parallel to the optical axis 50. Means (notshown) such as clamps are provided to secure or fix each of the lightsources 38, 40 in a given angular position.

In further contemplated embodiments, laser positioning units 54, 56,indicated schematically in phantom (FIG. 1), can be provided to changethe angular position of the light sources 38, 40, respectively, bymachine as opposed to by hand. Furthermore, angular encoders (not shown)can be provided to measure the angular positions of the light sources38, 40 and feed that information to the CPU or controller, and thatinformation can be used to control independently the angular position ofeach of the light sources.

Finally, with respect to the basic structure of the apparatus 10according to the invention, a light ring 60 preferably is provided. Thelight ring 60 is supported by arm 62, which is attached to either lasermounting bracket 20 or support arm 24. Light ring 60 is, for example, afiber optic bundle light ring attached to a halogen light source (notshown) and is used to illuminate the object being imaged, e.g.,calibration target 66, the use of which is described below. LED,fluorescent, and high frequency fluorescent lamps are alternatives.

Calibration target 66 is made from a flat piece of metal, such asaluminum, or plastic engraving material (i.e., a thin, flat layer ofblack plastic that is laminated to a white base layer of plastic) and iscut and marked with precise tolerance, e.a., 0.002 inch (0.05 mm)precision. Alternatively, calibration target 66 can be made by formingthe desired calibration pattern on photographic paper, e.g., by exposingand developing a sheet of film, or by forming the pattern from chromiumdeposited on glass for extremely precise tolerances.

A preferred calibration pattern for the calibration target 66 is shownin FIG. 3. Preferably, the surface 68 is primarily black, with themarkings being white (ie., the inverse of the pattern as shown in FIG.3). The markings include cross 70 in the form of a potent cross (seeMiriam Webster's Collegiate Dictionary, 10th Ed., illustration of“cross”), surrounded by rectangle 72. Preferably, the centralintersection portion of the cross 70 is “broken out” to facilitatealignment of the laser beams on the cross, as described below. Alignmenthashmarks 74 extend from the rectangle 72 to the edges of thecalibration target 66. As noted above, the calibration target is cut andmarked with a high degree of precision such that all markings are eitherperfectly parallel or perfectly perpendicular. Additionally, thedimensions of the markings—particularly the length, width, and markingthickness of the rectangle 72—are precisely known.

The calibration target 66 is used with the apparatus 10 to calibrate theapparatus as follows. Calibration target 66 is placed on supportplatform 14 beneath the camera 18, and the camera assembly is positionedat a starting height above the target which is, to some extent,arbitrary. The calibration target is generally centered and alignedwithin the field of view, preferably using a video overlay that isgenerated and superimposed on the video display/user interface either bythe frame grabber or by the video card driving the video display.Preferably, the image acquisition software program indicates the centerof the field of view and superimposes this location over the image beingshown to the user. The calibration target is then positioned on theplatform 14 such that the intersection 80 at the center of the cross 70lies at the center of the field of view, as can be observed by watchingthe monitor. The camera is then adjusted to bring the image of thecalibration target into focus and the image is acquired.

For certain applications, it may be desirable to use a fixed focuscamera—particularly for viewing objects at a specified, standardizedresolution. Even for such applications, however, it may still bedesirable to use extension tubes (with or without shims) or a back focusmechanism to provide fine tuning of the focus.

In a case where the camera 18 is a film-based photographic (still)camera, a picture of the calibration target is taken and developed. Thecalibration target markings are then measured as they appear on thephotograph and the resolution (scale) of the photographic image iscalculated by dividing the dimensions as shown in the photograph by theactual, known dimensions. The camera is then raised or lowered, eithermanually using knob 34 or, if the apparatus is so equipped, using drivemotor 36. This process is repeated until the desired resolution (scale)is obtained.

In the preferred embodiment, however, it is far easier to position thecamera such that a desired resolution is obtained. As noted above, thecamera 18 in the preferred embodiment is a Kodak® MASD MEGAPLUS™ 1.4 imodel digital video camera. With such a camera, an image of the objectbeing monitored, e.g., the calibration target 66, falls on a chargecoupled device (“CCD”). The CCD breaks the image down into a matrix(two-dimensional array) of square pixels, each pixel having anassociated x- and y-, Cartesian coordinate, and produces a digitalsignal consisting of light intensity information for each of the pixels.In the preferred embodiment, the picture (image) information from thecamera is passed to the CPU through the frame grabber, e.g., a Coreco®Oculus F/64™ model frame grabber available from Coreco Incorporated inSt. Laurent, Quebec, Canada, which compiles the digitized, pixilatedimage information a frame at a time. The digitized image information isthen analyzed by an appropriate software package designed to work withthe frame grabber information, e.g., (N) AFIS Acquire™, an imageacquisition/analysis and “machine vision” software package produced byAgris-Schoen Visions Systems, Inc. in Alexandria, Va.

The (N) AFIS Acquire™ software package is designed to work with thecalibration pattern shown in FIG. 3. The program searches the image forthe rectangle 72 and, once it locates the rectangle, identifies (butdoes not necessarily display) points A-H along the rectangle. By way ofexample, points A and B are each located one eighth of the length of theupper side of the rectangle (as shown in FIG. 3) from the left and rightsides of the rectangle, respectively. Similarly, points C and D are eachlocated one eighth of the length of the lower side of the rectangle fromthe left and right sides of the rectangle, respectively; and points Eand G and points F and H are located one eighth of the length of theleft and right sides of the triangle from the top and bottom sides ofthe rectangle, respectively. In general, the important feature is thatthe points in each pair of points located across the rectangle from eachother be the same distance from the respective side or top or bottomedges of the rectangle such that the line segment connecting them isparallel or perpendicular to all four edges of the rectangle.

Once the program locates points A-H, it determines the number of pixelsfrom point A to point C, point B to point D, point E to point G, andpoint F to point H by calculating the square root of the sum of thesquares of the x-coordinate and y-coordinate differences for the twopoints in each pair, i.e., by the Pythagorean theorem. The program thencalculates overall resolution by dividing the number of pixels frompoint A to point C by the known distance (precisely 1.9 inches (4.826cm)) from point A to point C; by dividing the number of pixels frompoint B to point D by the known distance (precisely 1.9 inches (4.826cm)) from point B to point D; by dividing the number of pixels frompoint E to point G by the known distance (precisely 2.4 inches (6.096cm)) from point E to point G; and by dividing the number of pixels frompoint F to point H by the known distance (precisely 2.4 inches (6.096cm)) from point F to point H and then averaging these four values.

In further contemplated embodiments, the program would be more flexibleand interactive, allowing the user to select (e.g., by pointing andclicking) points from which to calculate resolution. Such points couldbe offered as possibilities by the program, or they could be selected atthe user's discretion, assuming the user provides the program with theknown, actual distance between points—for example, if a ruler were beingused as the calibration target.

As noted above, the MASD MEGAPLUS™ 1.4 i model digital video cameraproduces an image with square pixels. If the pixels were rectangular,the program would calculate horizontal and vertical resolutionseparately by determining actual horizontal and vertical separation ofthe points in each pair (because the calibration target most likely willbe skewed slightly relative to the field of view) from the knowndistance between the points in each pair and taking account of the pixelaspect ratio, then dividing the number of pixels horizontally andvertically between the points in each pair by the actual horizontal andvertical separation of the points in each pair, respectively. Averagehorizontal and vertical resolution values then would be calculated byaveraging the values determined from each of the four point pairs A,C;B,D; E,G; and F,H; or from user-selected point pairs.

Once the system resolution is determined, it is displayed to the user,who raises or lowers the camera to adjust the resolution as necessary.The process is then repeated until the desired resolution is obtained.Preferably, the software has been provided with a desired value for theresolution—either by the user on a case-by-case basis or by theprogrammer for certain standardized applications—and the computerindicates to the user whether the camera should be raised or lowered toobtain the desired resolution.

Furthermore, it will be appreciated that if a drive motor 36 isprovided, particularly with position encoders, the CPU or controller cancontrol raising or lowering of the camera assembly automatically. Thecontrolling software would “know” to raise the camera if the number ofpixels per inch is too high, and to lower the camera if the number ofpixels per inch is too low. The amount by which the camera is raised orlowered (measured by the encoders) would be a function of the magnitudeof the resolution error.

Once the appropriate resolution has been obtained by adjusting (andsecuring, if desired) the height of the camera assembly, the lightsources 38, 40 are illuminated (if not already done so) and are pivotedsuch that the laser beams 46, 48 fall on and illuminate the calibrationtarget. The conventional, cylindrical beam laser light source is pivoteduntil the spot 82 of laser light formed thereby lies on the intersection80, i.e., in the immediate center of the field of view, as shown in FIG.4. At this point, that light source is locked in position such that itsangular orientation with respect to the camera is fixed.

The other light source, i.e., the line laser light source, is thenpivoted until the laser line 84 formed thereby is coincident with thedot 82 projected on the target by the first light source, as shown inFIG. 4, and preferably also is coincident with the alignment hashmarks74. (Using a line laser and a dot laser, as opposed to two dot lasers,facilitates this adjustment.) The second light source is then locked inposition so that its angular orientation with respect to the camera isfixed. The two beams 46, 48 thus will converge at the surface of thetarget 66 with the camera spaced from the target by a distance whichresults in the desired resolution, as shown in FIG. 2, preferably withthe beams intersecting in the center of the field of view.

In embodiments in which laser positioning units 54 and 56 are provided,the image acquisition software can be configured to identify the spot 82and line 84 formed by the laser light beams and can control the pivotingof the light sources 38, 40 to cause the beams to converge at the targetsurface.

At this point, the camera system can be used to image other objectshaving thicknesses or heights above the platform 14 different than thecalibration target 66, and to image such objects with the sameresolution easily and repeatably. Such an object (“the new object”) maybe, for example, a gun having a fingerprint which it is desired tophotograph and digitize for entry into and/or comparison against anationwide fingerprint database, or currency being analyzed ascounterfeit or legitimate.

The calibration target is removed and the new object is placed under thecamera; because the laser beams have been angled to intersect in thecenter of the field of view, it is easier to position the new objectgenerally centrally within the field of view simply by observing wherethe laser beams fall on the object. (Unless the new object is the sameheight as the calibration target, the laser beams no longer will becoincident at the surface of the object, but both beams still mostlikely will fall on the new object such that they can be used generallyto center it.) The camera is then raised or lowered as necessary which,because of the angled nature of the laser beams relative to the opticalaxis, causes the points at which the laser beams strike the surface ofthe new object to move as the camera is raised or lowered, and this canbe observed quite easily by watching the laser beams move across theobject. The camera is adjusted until the laser beams, which have notbeen re-angled relative to the camera, again converge at the surface ofthe object whose image is being acquired. It will be appreciated thatthe point of convergence of the laser beams will lie at the samelocation within the field of view of the camera as when the laser beamsconverged at the surface of the calibration target. Thus, at this pointthe resolution again will have the desired value.

The lasers are then turned off (temporarily) and the image of the objectbeing evaluated is acquired. Preferably, the lasers and the camera areinterlocked such that the lasers are automatically shut off while theimage is being acquired (whether a still or a video image) so as not tohave the acquired image include the laser beams and so as not tosaturate the acquired image with the laser light.

If the camera height adjustment is motorized, raising or lowering of thecamera to bring the laser beams back into convergence at the surface ofthe new object—i.e., to bring the resolution back to the desiredresolution—can also be controlled by the computer program. The programdetermines on which side of the laser line the laser dot lies, and fromthis determines whether the camera is too close to or too far from thenew object. The program then causes the camera to be raised or loweredaccordingly until the laser dot and laser line converge.

Although for most applications calibrating the camera using thegenerally flat, planar calibration target 66 provides excellent results,it is known that for any optical image acquisition system, there is aplane of optimal focus which is located generally in the center of thedepth of field, with the focus of the system “softening” fore and aft ofthe plane of optimal focus until the image would be deemed to be out offocus. For certain applications, it is desirable to have the camera lensfocus set such that the plane of optimal focus is coincident with theplane at which the resolution is the desired value.

To facilitate this, calibration block 100 has been developed, as shownin FIG. 5. The calibration block has at least one surface 102 which isoriented at a non-perpendicular, non-parallel angle α relative tocertain ones of the other surfaces, e.g., 45°. The surface 102 ispredominantly black, as indicated by shading, with a central “patch” ofwhite and a first calibration pattern 104 printed in the middle of thepatch of white, e.g., a “ladder” pattern as shown in FIG. 5A. The ladderpattern has two precisely spaced, perfectly parallel longitudinal lines106, and a series of evenly spaced, perfectly parallel cross-lines 108extending between the lines 106 like the rungs of a ladder.Additionally, the pattern has two center marks 110, perfectly parallelto the cross-lines 108, with one on either side of the pattern. Thepattern is positioned on the calibration block with the cross-lines 108extending parallel to the upper and lower edges 112, 114 of the surface102 (FIG. 5). Preferably, the size of the pattern is such that itextends beyond the bounds of the field of view of the camera when thepattern is imaged by the camera, at least in terms of the length of thepattern. This maximizes the benefit obtained with the calibration block.

When calibrating the camera, the calibration block 100 is placed on theplatform 14 below the camera, represented by the optical axis 50 in FIG.5. Assuming the field of view is rectangular, it is preferable for thecalibration block to be oriented such that the long dimension of thecalibration pattern 104 is aligned with the lengthwise direction of thefield of view, as indicated in FIG. 5B. Assuming the line laser lightsource is oriented such that the laser line projected thereby extendsvertically within the field of view, the laser line can be used tofacilitate this orientation by aligning it with the cross-lines 108 orthe two center marks 110. With this orientation, the line laser beamcrosses the surface 102 of the calibration block at a uniform distancefrom the line laser light source, rather than falling across portions ofthe surface 102 that are at different heights, thereby projecting asharper, more distinct laser line on the surface of the calibrationblock.

The angled nature of the surface 102 facilitates setting the plane atwhich the resolution is the desired value to be coincident with theplane of optimal focus as follows. Because the surface 102 is angled,the end of the pattern 104 that is closer to the top of the block willbe closer to the camera, and the opposite end will be further away fromthe camera. As a result, assuming the camera is adjusted to be focusedat a plane that lies between the horizontal plane passing through thetop of the pattern and the horizontal plane passing through the bottomof the pattern, the image of the pattern will vary from fuzzy and out offocus at the top of the pattern, to sharp and in focus somewhere in themiddle region of the pattern, and back to fuzzy and out of focus at thebottom region of the pattern, as shown in FIG. 5B. (It may be necessaryto adjust the size of the camera aperture (f-stop) to achieve thiscondition.)

Assuming the pattern 104 consists strictly of black lines on a whitebackground, the transition from black to white in the image, going fromone cross-line 108 to the next through the intervening space of“opposite” color, will be the sharpest, and therefore the most distinct,at the location of the plane of optimal focus. Accordingly, the softwareprogram analyzing the image information can locate the plane of optimalfocus by evaluating the differential in light intensity of adjacentpixels and finding the location where this differential is the greatest,corresponding to the sharpest transition from one cross-line to thenext. The software package will then display graphically to the user aline 120 (FIG. 5B), cutting across the image, corresponding to the linewhere the plane of optimal focus intersects the surface 102 of thecalibration block 100.

Next, the points P and Q (FIG. 5B) where the line 120 intersects theedges 107 of the black region/white patch are located by the software.The image analysis program calculates the resolution at the plane ofoptimal focus by determining the number of pixels between those twopoints and dividing by the distance between the edges 107, which isprecisely known and which has been provided to the program.

If the resolution matches the desired resolution, the system is ready tobe used to image subsequent objects. If, however, the resolution is notcorrect, the spacing between the camera and the lens is adjusted usingdifferent extension tubes, shims, or a back focus mechanism; the newlocation of the plane of optimal focus is found; and the resolution atthat plane is determined. (Simply raising or lowering the camera, as isdone when using the flat calibration target 66, would merely “pull” or“push” the plane of optimal focus up or down with the camera andtherefore would not change the resolution at the plane of optimalfocus.) This procedure is iterated until the plane of proper resolutioncoincides with the plane of optimal focus. It will be appreciated thatfocus and resolution are interrelated; accordingly, it may be necessaryto “play” with the system slightly to bring these two planes intocoincidence with the correct resolution. If the focus or back focusadjustment is computer-controlled and the height of the camera relativeto the calibration block is computer-controlled, the image analysissoftware can be configured to achieve this goal automatically.

To facilitate cross-checking the calibration, it is preferable toposition the pattern 104 on the surface 102 with center marks 110located a perpendicular distance h (FIG. 5) from the bottom surface 122of the block that is the same as the width W of the block. The camera iscalibrated by adjusting the height of it relative to the block such thatthe resolution at the horizontal plane passing through the marks 110 isthe desired resolution. Once the proper resolution and optimal focushave been achieved at the horizontal plane passing through the centeringmarks 110, the block is rotated and placed on its side such that surface130 faces upward and is exposed to the camera. Surface 130 has a secondpattern 132 which is identical to the calibration pattern on thecalibration target 66, shown in FIG. 3. Because the width w of thecalibration block is the same as the height h from the bottom surface122 to the centering marks 110, the pattern 132 will be positioned atthe same distance above the platform 14, when the block is positioned onits side, as the centering marks 110 were. The resolution of the systemis then verified by determining the resolution using the second patternin the manner described above.

Once the resolution is verified, the focus and/or back focus are lockedand all parameters are retested to confirm that they have not beenchanged, and the lasers are angled relative to the camera and locked inposition, as described above. This may be done using the secondcalibration pattern 132, identically to the method described above.Alternatively, the calibration block could be repositioned so that it isoriented as shown in FIG. 5. In that case, the line laser would beangled such that the line projected thereby passes through the center ofthe field of view (determined by observing the video overlay on themonitor); the calibration block would be positioned such that the laserline passes through the two centering marks 110; and then theconventional laser would be angled such that the laser dot falls on thelaser line at the surface 102 and generally in the center of the fieldof view (also determined by observing the video overlay on the monitor).

The calibration block 100 is then removed from the platform 14 and thecamera system is used to image other objects in the same mannerdescribed above.

Finally, it will be appreciated that the invention admits of numerousalterations and modifications, all of which are deemed to be within thescope of the invention. For example, although the apparatus and methodhave been described in connection with a camera located above and aimeddown on a target, the system can be constructed so as to be orientedhorizontally and/or to be portable for field use, as shown in FIGS. 6Aand 6B, wherein elements with primed reference numbers correspondgenerally to the elements described above with non-primed referencenumbers. In this embodiment, bracket 14′ is configured to hold thecalibration target 66 for alignment of the light sources 38′, 40′ in themanner described above. The bracket 14′ and optics can be pre-configuredfor a given resolution, or an overlay in the view finder (or on thevideo monitor) can be used to align the target for a given resolution.Alternatively, the target can be set at any distance (and hence anyresolution) and the lights aligned with it. The resolution can then bedetermined either in the field or later, depending upon availableequipment.

The calibration target 66 is then removed and the camera is used toacquire the image of an object of interest, maintaining the camera atthe proper distance from the object (and hence resolution) by ensuringthat the beams 46′, 48′ converge at the surface of the object ofinterest. The camera 18′ and laser mounting bracket 20′ can be removedfrom the support bracket 14′ and used to acquire the image of an objectat any location without being hindered by the support bracket 14′, orthe assembly can be left intact, with the support bracket 14′ being usedas a resting device or frame for the object of interest.

If the camera is being held by hand, it may be desirable to add a thirdor even a fourth laser, adjusting the additional laser(s) such that thebeam(s) converge(s) at the surface of the calibration target either withthe beams of the first two lasers or, if four lasers are being used,with each other's beam. This facilitates orienting the camera such thatthe optical axis is normal to the plane of the surface of the objectbeing imaged.

In another alternative embodiment, which provides increased accuracy ofthe alignment of the calibration target, calibration block, orsubsequent object of interest being imaged, the support platform 14″ isconstructed with an x-y-θ positioning apparatus 200. The positioningapparatus has a first track 202 (which may be recessed in the platform14″ such that its top surface is level with the surface of the platform14″); a second track 204 which is perfectly perpendicular to the firsttrack 202 and which slides back and forth along the track 202; and anangular orientation table 206, which slides back and forth along thesecond track 204. The angular orientation table has an upper pallet 208on which the object being imaged is supported and which itself issupported by and rotates relative to support block 210. The linear andangular position of each of these elements can be controlled with agreat degree of precision by means (not shown) which are known andavailable to those having skill in the art.

Furthermore, although the light sources 38, 40 are shown and describedas being positioned such that both of their beams are offset from andangled relative to the optical axis 50, it will be appreciated that thesystem can be configured such that one of the beams is coincident withthe optical axis over at least a portion of the optical axis. This maybe achieved by placing a beam splitter in between the camera and thecalibration target or other object being imaged, with the beam splitteroriented to allow the camera to acquire the image of the target orobject, and shining a laser at the beam splitter from the side such thatthe beam from that laser is reflected by the beam splitter down alongthe optical axis 50. The benefit of such an arrangement is that onelight source will always illuminate the object at the center of thefield of view; the disadvantage (as compared to the configurationsdescribed above) is that the accuracy and ease of adjustment provided byhaving two beams converge or diverge as the camera is moved toward oraway from the object being viewed is diminished by fifty percent.

Finally, in a variant of this last-described alternative embodiment,calibration and repeated setting of the resolution of the system can beachieved using only a single offset light source if the location of thecenter of (or some other fixed location within) the camera's field ofview is displayed to the user, e.g., using a video overlay. The userwould adjust the camera to obtain the desired resolution, and then thesingle light source would be pivoted until its beam illuminates thecalibration target precisely in the center of (or at the other markedspot within) the field of view. (A spot laser or other dot-producinglight source should be used; a line laser would not work as well forthis embodiment.) Then, the proper resolution could be required, forimaging any other object, by raising or lowering the camera until thebeam illuminates the object at a point that is at the marked, referenceposition in the field of view.

Further embodiments having other modifications and improvements aredeemed to be within the scope of the following claims.

We claim:
 1. A high-precision-resolution image acquisition apparatus,said apparatus comprising: a support surface for supporting an object tobe imaged; a camera supported at an optical distance from said supportsurface for acquiring an image of an object being imaged, said cameraoutputting a signal representing the image of said object being imagedand said camera having an optical path along which said image isacquired and a field of view, said image having a resolution and saidoptical distance being adjustable such that the resolution of said imagecan be adjusted; computer means for analyzing said signal to determinethe resolution of said image; a first light source which projects afirst beam of light so as to strike said object being imaged at a firstlocation on a surface of said object being imaged, said first lightsource being configured such that said first location varies as saidoptical distance is adjusted; and a second light source which projects asecond beam of light so as to strike said object being imaged at asecond location on said surface of said object being imaged, wherein oneof said first and second light sources is a line laser and the other ofsaid first and second light sources is a cylindrical beam laser whichprojects a dot or spot of laser light on said object being imaged. 2.The apparatus of claim 1, wherein said first light source is configuredsuch that said first beam of light is angled relative to said opticalpath whereby said first location varies as said optical distance isadjusted.
 3. The apparatus of claim 1, wherein said second light sourceis configured such that said second beam of light is angled relative tosaid optical path whereby said second location varies as said opticaldistance is adjusted.
 4. The apparatus of claim 1, wherein said firstand second light sources are configured such that said first and secondbeams of light are angled relative to said optical path whereby saidfirst and second locations vary as said optical distance is adjusted. 5.The apparatus of claim 4, wherein said first and second light sourcesare spaced from said camera and are pivotable relative to said camerawhereby said first and second beams of light can be caused to intersectat said surface of said object being imaged such that said first andsecond locations are coincident.
 6. The apparatus of claim 1, whereinsaid computer means comprise computer hardware.
 7. The apparatus ofclaim 1, wherein said camera is a video camera and said computerhardware comprises a frame grabber.
 8. The apparatus of claim 1, whereinsaid computer means comprise computer software.
 9. The apparatus ofclaim 1, wherein said signal comprises a pixilated representation ofsaid image and wherein said resolution is expressed in terms of pixelsper unit of length of said object being imaged.
 10. The apparatus ofclaim 1, further comprising a motor which causes said optical distanceto be adjusted.
 11. The apparatus of claim 10, wherein said computermeans controls operation of said motor to cause said optical distance tobe adjusted such that said resolution has a desired value.
 12. Theapparatus of claim 10, said apparatus further comprising computer meansfor analyzing said signal to determine said first location, saidcomputer means controlling operation of said motor to cause said opticaldistance to be adjusted such that said first location is at a desiredposition.
 13. The apparatus of claim 1, further comprising a motor whichcauses said optical distance to be adjusted.
 14. The apparatus of claim13, said apparatus further comprising computer means for analyzing saidsignal to determine said first and second locations, said computer meanscontrolling operation of said motor to cause said optical distance to beadjusted such that said first and second locations are coincident. 15.The apparatus of claim 1, further comprising a support platform on saidsupport surface, said support platform permitting precise control of thelinear and angular positioning of said object being imaged.
 16. Theapparatus of claim 1, wherein said first light source and said cameraare interlocked such that said first light source does not project saidfirst beam of light as said image is being acquired.
 17. Ahigh-precision-resolution image acquisition apparatus, said apparatuscomprising: a support surface for supporting an object to be imaged; acamera supported at an optical distance from said support surface foracquiring an image of an object being imaged, said camera outputting asignal representing the image of said object being imaged and saidcamera having an optical path along which said image is acquired and afield of view, said image having a resolution and said optical distancebeing adjustable such that the resolution of said image can be adjusted;computer means for analyzing said signal to determine the resolution ofsaid image; and a first light source which projects a first beam oflight so as to strike said object being imaged at a first location on asurface of said object being imaged, said first light source beingconfigured such that said first location varies as said optical distanceis adjusted; wherein said first light source is configured such thatsaid first beam of light is angled relative to said optical path wherebysaid first location varies as said optical distance is adjusted, saidapparatus further comprising a first motor which controls the angle ofsaid first beam of light relative to said optical path.
 18. Theapparatus of claim 17, said apparatus further comprising computer meansfor analyzing said signal to determine said first location, saidcomputer means controlling operation of said first motor to cause saidfirst location to be adjusted to a desired position.
 19. The apparatusof claim 18, wherein said desired position is the center of the field ofview of said camera.
 20. A high-precision-resolution image acquisitionapparatus, said apparatus comprising: a support surface for supportingan object to be imaged; a camera supported at an optical distance fromsaid support surface for acquiring an image of an object being imaged,said camera outputting a signal representing the image of said objectbeing imaged and said camera having an optical path along which saidimage is acquired and a field of view, said image having a resolutionand said optical distance being adjustable such that the resolution ofsaid image can be adjusted; computer means for analyzing said signal todetermine the resolution of said image; a first light source whichprojects a first beam of light so as to strike said object being imagedat a first location on a surface of said object being imaged, said firstlight source being configured such that said first location varies assaid optical distance is adjusted; a second light source which projectsa second beam of light so as to strike said object being imaged at asecond location on said surface of said object being imaged; and a firstmotor which controls the angle of said first beam of light relative tosaid optical path.
 21. The apparatus of claim 20, said apparatus furthercomprising computer means for analyzing said signal to determine saidfirst and second locations, said computer means controlling operation ofsaid first motor to cause said first location to be adjusted to adesired position, said desired position being coincident with saidsecond location.
 22. The apparatus of claim 21, wherein said desiredposition is at the center of the field of view of said camera.
 23. Theapparatus of claim 21, wherein said second light source is configuredsuch that said second beam of light is angled relative to said opticalpath whereby said second location varies as said optical distance isadjusted, said apparatus further comprising a second motor whichcontrols the angle of said second beam of light relative to said opticalpath, said computer means controlling operation of said first and secondmotors to cause said first and second locations to be adjusted to becoincident.
 24. A method of acquiring the image of an object with arepeatable resolution, said method comprising the steps: providing afirst object; providing a camera at a first optical distance from saidfirst object, said camera having a field of view and an optical pathalong which an image is acquired; positioning said first object withinsaid field of view; providing a first light source and, with said firstlight source, projecting a first beam of light so as to strike saidfirst object at a first location on a surface of said first object, saidfirst light source being configured such that said first location variesas said first optical distance is adjusted; providing a second lightsource and, with said second light source, projecting a second beam oflight so as to strike said first object at a second location on saidsurface of said first object; adjusting said first light source to causesaid first location to be coincident with said second location wherebysaid first and second beams of light converge at the surface of saidfirst object; removing said first object from the field of view;providing a second object within said field of view at a second opticaldistance from said camera, said first and second beams of light stirringsaid second object at third and fourth locations, respectively, on asurface of said second object; adjusting said second optical distance,if necessary, to cause said third and fourth locations to be coincidentwithout repositioning said first light source, said second light source,or said camera relative to each other, whereby said first and secondbeams of light converge at the surface of said second object; andacquiring an image of said second object, said method further comprisingbefore adjusting said first light source, 1) acquiring an image of saidfirst object, said image of said first object having a resolution; 2)determining said resolution; and 3) adjusting said first opticaldistance, if necessary, to cause said resolution to have a desiredvalue.
 25. The method of claim 24, wherein said image of said firstobject is pixilated and wherein said resolution is expressed in terms ofpixels per unit of length of said first object.
 26. The method of claim24, wherein said camera is a film-based camera and said resolutioncomprises a dimension of said first object as shown in said image ofsaid first object divided by a corresponding actual dimension of saidfirst object.
 27. The method of claim 24, further comprising removingsaid second object from the field of view; providing a third objectwithin said field of view at a third optical distance from said camera;adjusting said third optical distance, if necessary, to cause said firstand second beams of light to converge at the surface of said thirdobject; and acquiring an image of said third object.
 28. A method ofacquiring the image of an object with a repeatable resolution, saidmethod comprising the steps: providing a firs t object; providing acamera at a first optical distance from said first object, said camerahaving a field of view, an optical path along which an image isacquired, a lens, and a medium on which said image falls; positioningsaid fit object within said field of view; providing a first lightsource and, with said first light source, projecting a first beam oflight so as to strike said first object at a first location on a surfaceof said first object, said first light source being configured such thatsaid first location varies as said first optical distance is adjusted;providing a second light source and, with said second light source,projecting a second beam of light so as to strike said first object at asecond location on said surface of said first object; adjusting saidfirst light source to cause said first location to be coincident withsaid second location whereby said first and second beams of lightconverge at the surface of said first object; removing said first objectfrom the field of view; providing a second object within said field ofview at a second optical distance from said camera, said first andsecond beams of light striking said second object at third and fourthlocations, respectively, on a surface of said second object; adjustingsaid second optical distance, if necessary, to cause said third andfourth locations to be coincident without repositioning said first lightsource, said second light source, or said camera relative to each other,whereby said first and second beams of light converge at the surface ofsaid second object; and acquiring an image of said second object; saidmethod further comprising before adjusting said first light source, 1)acquiring an image of said first object, said image of said first objecthaving a resolution; 2) determining said resolution; and 3) adjusting adistance between said lens and said medium, if necessary, to cause saidresolution to have a desired value.
 29. The method of claim 28, whereinsaid image of said first object is pixilated and wherein said resolutionis expressed in terms of pixels per unit of length of said first object.30. The method of claim 28, wherein said camera is a film-based cameraand said resolution comprises a dimension of said first object as shownin said image of said first object divided by a corresponding actualdimension of said first object.
 31. A method of acquiring the image ofan object with a repeatable resolution, said method comprising thesteps: providing a first object; providing a camera at a first opticaldistance from said first object, said camera having a field of view andan optical path along which an image is acquired; positioning said firstobject within said field of view; providing a first light source and,with said first light source, projecting a first beam of light so as tostrike said first object at a first location on a surface of said firstobject, said first light source being configured such that said firstlocation varies as said first optical distance is adjusted; providing asecond light source and, with said second light source, projecting asecond beam of light so as to strike said first object at a secondlocation on said surface of said first object; adjusting said firstlight source to cause said first location to be coincident with saidsecond location whereby said first and second beams of light converge atthe surface of said first object; removing said first object from thefield of view; providing a second object within said field of view at asecond optical distance from said camera, said first and second beams oflight striking said second object at third and fourth locations,respectively, on a surface of said second object; determining whether toincrease or decrease said second optical distance based on the relativepositions of said third and fourth locations on said surface of saidsecond object; adjusting said second optical distance, if necessary, tocause said third and fourth locations to be coincident withoutrepositioning said first light source, said second light source, or saidcamera relative to each other, whereby said first and second beams oflight converge at the surface of said second object; and acquiring animage of said second object.
 32. A method of acquiring the image of anobject with a repeatable resolution, said method comprising the steps:providing a first object; providing a camera at a first optical distancefrom said first object, said camera having a field of view a and anoptical path along which an image is acquired; positioning said firstobject within said field of view; providing a first light source and,with said first light source, projecting a first beam of light so as tostrike said first object at a first location on a surface of said firstobject, said first light source being configured such that said firstlocation varies as said first optical distance is adjusted; providing asecond light source and, with said second light source, projecting asecond beam of light so as to strike said first object at a secondlocation on said surface of said first object; adjusting said firstlight and second light sources to cause said first location to becoincident with said second location whereby said first and second beamsof light converge at the surface of said first object; removing saidfirst object from the field of view; providing a second object withinsaid field of view at a second optical distance from said camera, saidfirst and second beams of light striking said second object at third andfourth locations, respectively, on a surface of said second object;adjusting said second optical distance, if necessary, to cause saidthird and fourth locations to be coincident without repositioning saidfirst light source, said second light source, or said camera relative toeach other, whereby said first and second beams of light converge at thesurface of said second object; and acquiring an image of said secondobject.